Ion exchange membranes are used as solid electrolytes in electrochemical energy conversion devices such as fuel cells, electrolysers, in separation processes, sensors, etc. Perfluorinated ion exchange membranes are state of the art proton exchange membranes (PEM) used as electrolytes for fuel cells. Polymer Electrolyte Membrane Fuel Cell (PEMFC) technologies are efficient energy conversion devices where the PEM used as the electrolyte play a central role serving as both electrolyte and gas separator. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEMFC must be robust, highly proton conductive, and gas impermeable. Such membranes are generally prepared by solution or dispersion casting of the acid form of the ionomer or by melt-extrusion of a precursor polymer containing sulfonyl halide protective groups that has to be hydrolyzed and acid-exchanged after melt-processing.
There is still a continuing need to reduce cost and the level of manufacturing processes complexity while improving durability and reliability of PEM. To date, solution-cast perfluorosulfonic acid (PFSA) ionomer membranes (e.g. Nafion™) and solution-cast PFSA membranes reinforced with polytetrafluoroethylene (PTFE) support (e.g. Gore®) have been the most widely used membranes for PEMFCs. However, these materials are costly and still need to meet the requirements for high volume commercial markets. The mechanical and chemical durability of proton exchange membranes is also essential for building robust and long-lasting PEM fuel cells for automotive and stationary applications.
Melt processes represent the best technologies for mass production of homogeneous thin polymer films at low cost. Besides eluding the serious safety and environmental concerns related to the mass production of membranes by solution-casting, melt processes provide a mechanical reinforcement through chain orientations following extrusion-stretching. This structural reinforcement at a molecular level provides extruded PEM with the mechanical durability required for building robust and long-lasting PEM fuel cells. Extruded membranes have already proven to have a much higher mechanical and chemical durability in a fuel cell than solution-cast membranes (Lai 2009). In situ humidity cycling experiments, designed to assess the mechanical durability in PEM, have demonstrated that the stresses induced in the membrane upon cycling between wet and dry conditions can lead to crack formation, which leads to gas crossover and ultimately failure of the fuel cell.
Extruded PEMs are generally processed from non-ionomeric (non-conducting) polymers that have to go through a post-functionalization reaction (e.g. post-sulfonation) to introduce ionic functionalities or from precursors where the ionic groups are protected (e.g. PFSAs in the sulfonyl fluoride form). These non-functional analogs are then chemically converted into their functional counterparts (e.g. their acid form). In both cases, several chemical treatments have to be conducted during the manufacturing process prior to catalytic layers integration, which adds complexity to an otherwise simple process. It is highly desirable to directly extrude the functional polymers in their active form. Unfortunately, the strong ionic associations in ionomers act generally as physical cross-links, increasing by several orders of magnitude both melt-viscosities and relaxation times, resulting in ionomeric materials that are very difficult to melt-process. The strength of the ionic interactions in ionomers, and hence their physical and mechanical properties, depends on the acidity of the pendent anion. Polymers modified with the stronger acid, such as sulfonic acid (pKa about 1), exhibit more dramatic changes in thermal, viscoelastic, and rheological properties than those modified with the weaker carboxylic acid (pKa about 4-5). However, most ionomers may be melt-processed in very low shear rate operations such as compression-molding, which reveals that these ionic associations are not permanent cross-links and can be reversibly disrupted under suitable conditions.
It is known in the art (Sanchez 2009 for example) that plasticizers like imidazole, N-alkylimidazole and N-vinylimidazole can be used as processing aids for melt processing of polysulfone polymers. However, these processing aids are particular to polysulfone-like polymers and are not transferable to the melt-processing of PFSA ionomers.
It is further known (Sen 2008) that 1H-1,2,4-triazole can be used to improve anhydrous proton conduction of Nafion™ membranes, but the triazole is introduced into these membranes by solution impregnation techniques, not melt-processing techniques. There is no suggestion that the triazole can be used as a plasticizer in a melt-processing process. The resulting poor dispersion of the triazole in the film detracts from the mechanical properties of the membrane.
There remains a need for PFSA-based ion exchange membranes with improved properties.